Point-to-point reliable delivery between a single producer actor sending messages and a single consumer actor receiving the messages. Used together with ConsumerController.

The producer actor will start the flow by sending a ProducerController.Start message to the ProducerController. The ActorRef in the Start message is typically constructed as a message adapter to map the ProducerController.RequestNext to the protocol of the producer actor.

For the ProducerController to know where to send the messages it must be connected with the ConsumerController. You do this is with ProducerController.RegisterConsumer or ConsumerController.RegisterToProducerController messages.

The ProducerController sends RequestNext to the producer, which is then allowed to send one message to the ProducerController via the sendNextTo in the RequestNext. Thereafter the producer will receive a new RequestNext when it's allowed to send one more message.

The producer and ProducerController actors are supposed to be local so that these messages are fast and not lost. This is enforced by a runtime check.

Many unconfirmed messages can be in flight between the ProducerController and ConsumerController. The flow control is driven by the consumer side, which means that the ProducerController will not send faster than the demand requested by the ConsumerController.

Lost messages are detected, resent and deduplicated if needed. This is also driven by the consumer side, which means that the ProducerController will not push resends unless requested by the ConsumerController.

Until sent messages have been confirmed the ProducerController keeps them in memory to be able to resend them. If the JVM of the ProducerController crashes those unconfirmed messages are lost. To make sure the messages can be delivered also in that scenario the ProducerController can be used with a DurableProducerQueue. Then the unconfirmed messages are stored in a durable way so that they can be redelivered when the producer is started again. An implementation of the DurableProducerQueue is provided by EventSourcedProducerQueue in pekko-persistence-typed.

Instead of using tell with the sendNextTo in the RequestNext the producer can use context.ask with the askNextTo in the RequestNext. The difference is that a reply is sent back when the message has been handled. If a DurableProducerQueue is used then the reply is sent when the message has been stored successfully, but it might not have been processed by the consumer yet. Otherwise the reply is sent after the consumer has processed and confirmed the message.

If the consumer crashes a new ConsumerController can be connected to the original ProducerConsumer without restarting it. The ProducerConsumer will then redeliver all unconfirmed messages.

It's also possible to use the ProducerController and ConsumerController without resending lost messages, but the flow control is still used. This can for example be useful when both consumer and producer are know to be located in the same local ActorSystem. This can be more efficient since messages don't have to be kept in memory in the ProducerController until they have been confirmed, but the drawback is that lost messages will not be delivered. See configuration only-flow-control of the ConsumerController.

The producerId is used in logging and included as MDC entry with key "producerId". It's propagated to the ConsumerController and is useful for correlating log messages. It can be any String but it's recommended to use a unique identifier of representing the producer.

If the DurableProducerQueue is defined it is created as a child actor of the ProducerController actor. It will use the same dispatcher as the parent ProducerController.